- We add BackgroundSchedulePool which can execute a function at a specific point in time. Basically all tasks are added in a queue and precessed by worker threads.
- The most important difference between this and BackgroundProcessingPool is that we have the guarantee that the same function is not executed from many workers in the same time
- Each of the following classes instead starting a thread will register a task in BackgroundSchedulePool and when they need to run will call schedule or scheduleAfter(duration) functions
This commit is moving all threads created by ReplicatedMergeTree to BackgroundSchedulePool tasks
NOTE: I did a minimum number of changes to be much simple to review the code
